1. Technical Field
The present invention relates to saving water, wastewater, lime, and energy while increasing productivity and enhancing product quality in a nixtamalization process and, more particularly, to a method for saving water whereby supernate from a corn-slurry is not drained, but is instead reused in a subsequent batch thereby saving precious resources. The method of this invention also relates to a method for saving water by reusing some water in such a manner as to avoid impurities.
2. Description of Related Art
The corn from corn tortilla chips such as those in the snack food industry is sometimes cooked and soaked prior to being made into a flour, dough, or masa. One example of this process is the treatment of corn in a nixtamalization process—the traditional method for processing fresh corn to form masa dough. This process dates back to the pre-Columbian era of the Aztec and Maya people in Mesoamerica. In the traditional nixtamilization process, fresh whole-kernel corn is first soaked in a solution of water and lime (calcium hydroxide) and then partially cooked at or near the boiling point for a short time depending on the hardness of the corn. The corn is then steeped in the limewater solution and is allowed to cool for about 8-18 hours in order to loosen and degrade the pericarp (or bran), which is the outer, fibrous layer of a corn kernel. Cooking and steeping in alkaline solution causes partial dissolution of the cuticle and other pericarp layers as well as swelling and weakening of cell walls and fiber components. The corn kernels are then drained of the cooking liquor (called “nejayote”), which contains loosened pericarp and other dissolved or suspended particles, and the corn kernels are washed to remove excess lime and loose particles. Typically, up to 15% by weight of the total corn fraction is lost during the cooking and washing steps. The corn kernels are then ground to disrupt the starch-containing cell structures and cause the mixture to gelatinize. The ground, wet mixture can be mixed with water to form fresh masa dough, or it can be dehydrated to form dry masa flour. Dry masa flour can be rehydrated at a later time to form masa dough.
A schematic diagram showing a prior art cook and soak process is illustrated in FIG. 1. In this prior art process, two kettles are used. The process starts with the addition of various components of an aqueous alkaline and grain mixture. For example, 1200-1700 pounds of corn, 1-5% lime, and about 325 gallons of water are placed into a steam-jacketed kettle 10. This mixture is then heated to its cook temperature by use of a steam jacket to near boiling.
Once the target temperature is reached, the corn, lime, and water mixture is cooked at the cook temperature for a set number of minutes. Following the cook, 325 gallons of fresh water 24 is added to the kettle 10 via a fresh water line 11 to cool the batch approximately about 60-80° F. In this prior art example, approximately 650 gallons of water is required for every batch of corn processed through the kettles 10. The kettle of corn-slurry is then pumped via a discharge line 12 to a soak tank 14 to be “steeped” or soaked. Some of the soak tank excess water is drained via an overflow line 16 to the sewer 60 or other discharge processing means. In many instances, water discharged to the sewer is fairly alkaline and must be treated as wastewater prior to its final discharge into the environment. After the corn-slurry has been in the soak tank 14 for about 8-18 hours, the slurry is sent to a corn hopper 30 by way of a transfer line 18. Although FIG. 1 shows only two soak tanks, because the time the corn is in the soak tank 14 exceeds the time the corn is in the kettle 10, many more soak tanks 14 may be required if a continuous process is desired. A fresh push water stream 22 of about 10-20 gallons per minute is sprayed into the soak tank 14 discharge pipe to transport the corn from the soak tank 14. About 90 gallons of water is typically used to transport the corn. The corn hopper 30 separates corn and water. The excess water is sent to the corn hopper drain line 34 and then to the sewer 60. The corn-slurry is then gravity fed via a hydrosieve 32 into a washer 38. Excess water is sent to the hydrosieve drain line 36 and then to the sewer 60. The washer 38 is a rotating drum that utilizes a fresh wash water stream 26 to gently rinse the corn of hulls and lime. The fresh wash water stream 26 typically flows at 20-30 gallons per minute. The excess washer wastewater stream 40 from the washer 38 is also drained and routed to the sewer 60. From the washer 38, the corn is sent to the drain belt 42 where the excess water, via the drain belt wastewater stream 46, is sent to the sewer 60. The corn is then sent to further processing 44 where it is made into a product such as flour, masa, or dough.
There are several drawbacks to this process. One drawback is that fresh kettle water, usually at ambient temperature must be heated via a steam jacket in the kettles 10 to a temperature at or near boiling, which requires substantial resources. Another drawback is that there is an undesirable temperature gradient in the kettle 10. Because ambient water in the center of the kettle 10 comes in much cooler as fresh kettle water and because mixing in the kettle is done in a very gentle manner, the temperature gradient results in some uneven cooking, lessening product quality. An additional drawback is that any soak tank excess water 16, after being heated, is then drained and sent to the sewer via the overflow line 16. Thus, the process has spent resources heating water that is essentially wastewater. Additionally, as previously mentioned, these waste streams are often discharged with relatively high alkalinity and solids contents that often require further treatment prior to their discharge back into the environment. Consequently, there is a need to minimize the amount of soak tank excess water that is discharged from the soak tanks 14 to save water, steam, and wastewater treatment resources that does not diminish productivity or product quality.
One prior art method used to solve this is illustrated by U.S. Pat. No. 3,117,868 issued to Madrazo on Jan. 14, 1964. Rather than cooking the corn in a saturated limewater solution, the Madrazo patent discloses mixing hot alkaline water vapors with the corn in a pressurized vessel. This invention has a major drawback, however. It would require the purchase and installation of major pieces of new equipment, namely a pressure vessel and perhaps a blower, depending on the embodiment of the invention chosen.
Another drawback to the prior art grain cook and steep process depicted in FIG. 1 is that fresh push water 22 must be used to cool and transport the corn and its associated transfer line 18 only to be later drained in either the corn hopper drain line 34 or the hydrosieve drain line 36. The fresh push water stream 22 typically flows at 10-20 gallons per minute.
Recycling wastewater streams is not easily achieved because solids build-up from the corn can occur. Solids build-up is undesirable because it causes lime blinding of the hydrosieve 32 and washer 38 screens as well as fouling and scaling of the equipment that requires downtime to clean, lowering production rates. The presence of solids also increases the presence of microbes—an undesirable result in the food processing industry.
Fresh wash water is also used only once to rinse the corn before becoming washer wastewater stream 40. The fresh wash water stream 26 typically flows at about 20-30 gallons per minute. Again, this wastewater stream is often discharged with relatively high alkalinity and solids contents that require further treatment prior to their discharge back into the environment.
Consequently a need exists for methods to minimize amounts of water, wastewater and steam required that can be adapted to existing processing equipment while maintaining or enhancing production rates and product quality.